5′-guanylic acid (GMP) can be produced by microbial enzymatic or chemical breakdown of yeast RNA, direct fermentation for directly producing nucleotides using mutant strains cultured in media containing a sugar, a nitrogen source, and a phosphate source, or a combination method including the fermentative synthesis of nucleotide intermediates and the chemical or enzymatic conversion of the nucleotide intermediates to nucleotides. In recent years, a combination method that combines fermentative or chemical synthesis and enzymatic conversion is industrially widely used due to its economical advantages.
GMP production by the combination method consists of fermentative synthesis of 5′-xanthylic acid (XMP) and enzymatic conversion of XMP to GMP. At this time, two types of microbial strains are used. That is, an XMP-producing strain is used in the fermentative synthesis, and a microbial strain capable of inducing a continuous high-level expression of a gene encoding XMP aminase activity is used in the enzymatic conversion. In this case, the survival rate of the microbial strain used in the enzymatic conversion is remarkably reduced during a later culture period relative to during an initial culture period, thereby decreasing the production of GMP. Furthermore, it is known that the guaA gene encoding XMP aminase is fatal to host cells, and thus, the continuous high-level expression of XMP aminase hinders the growth of the microbial strain used in the enzymatic conversion during the later culture period. In addition, it is known that an internal gene responsible for GMP degradation in the microbial strain used for the enzymatic conversion is expressed, thereby facilitating the degradation of GMP.
In view of the above problems, the present inventors developed Escherichia sp. GPD1114 (Accession No. KCCM-10543) including an XMP aminase-encoding gene capable of expressing XMP aminase according to cellular growth state and an inactivated internal glnL gene. Furthermore, the present inventors developed Escherichia sp. GPU1114 (Accession No. KCCM-10536) derived from the Escherichia sp. GPD1114, in which the ushA gene encoding 5′-nucleotidase is inactivated to remove GMP degradability.
However, since the Escherichia sp. GPU1114 still contains internal genes associated with GMP degradation except the ushA gene encoding 5′-nucleotidase, GMP can be degraded into guanosine or guanine. Therefore, a microbial strain with simultaneous inactivation of all genes associated with GMP degradation is required in the art.